Booking announced the proposed acquisition of private equity firm CVC Capital Partners-owned Etraveli in November last year.

Etraveli offers search, booking and fulfilment services of flights through its consumer brands Gotogate and Mytrip and also operates airline integration company TripStack.

The European Commission, which has been assessing the deal since Oct. 10, will open an in-depth investigation at the end of its preliminary review on Nov. 16 because of competition concerns, the people said.

Booking had until Wednesday to offer remedies but did not do so, a filing on the Commission website showed on Thursday.

The EU competition watchdog declined to comment.

Booking said: "We have been and continue to work closely with the European Commission."

Britain's antitrust agency cleared the deal unconditionally in September.

($1 = 0.9829 euros)

(Reporting by Foo Yun Chee;Editing by Elaine Hardcastle)

By Foo Yun Chee